Calibrating Vascular Plant Abundance for Detecting Future Climate Changes in Oregon and Washington, USA
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Calibrating vascular plant abundance for detecting future climate changes in Oregon and Washington, USA a * b b Timothy J. Brady : , Vicente J. Monleon , Andrew N. Cray ABSTRACT We propose using future vascular plant abundances as indicators of future climate in a way analogous to the reconstruction of past environments by many palaeoecologists. To begin monitoring future short- term climate changes in the forests of Oregon and Washington. USA. we developed a set of transfer functions for a present-day calibration set consisting of climate parameters estimated. and species abundances measured. at 107 USDA Forest Service FIA(Forest Inventory and Analysis) Phase 3 plots. For each plot. we derived climate estimates from the Daymet model database. and we computed species abundance as quadrat frequency and subplot frequency. We submitted three climate variables (mean January temperature. MJAT; mean July temperature. MJUT; and mean annual precipitation transformed to natural logarithms. MANPt) to canonical correspondence analysis (CCA). and verified their importances in structuring the species frequency data. Weighted averaging-partial least squares regression (WA-PLS) provided the means for calculating six transfer functions. In all cases. based on performance statistics generated by leave-one-out cross-validation. we identified two-component WA- PLS models as the most desirable. The predictive abilities of our transfer functions are comparable to. or better than. those reported in the literature. probably due both data quality and statistical considerations. However. model overfitting as a result of spatial autocorrelation remains a possibility. The large errors associated with our MJAT transfer functions connote that even the highest amount of change in mean January temperature predicted for Oregon and Washington for 2010-2039 would be indistinguishable from current conditions. The higher predictions indicate that our MJAT transfer functions may be able to track climate changes by the 2040s. Our MJUT transfer functions can detect change in mean July temperature under the highest projection for 2010-2039. Our MANPt transfer functions will be of limited use until the 2070s. given the predictions of only slight changes in mean annual precipitation during the early part of the twenty-first century. Our MJAT and MANPt transfer functions may prove useful at the present time to verify relative climatic stability. Because the predicted climate values sometimes deviate substantially from the observed values for individual plots. our transfer functions are appropriate for monitoring climatic trends over the entire Pacific Northwest or large regions within it. not for assessing climate change at individual plots. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction species as indicators potentially offers very high-resolution data on local climate where instrumentation is unavailable (e.g., in remote Climate Significantly affects the growth rates and equilibrium mountainous locations). Given the recent and expected future rapid abundances of populations of vascular plants. and therefore. it climate change (e.g ., IPCC, 2007). decision makers demand high influences their evolutionary histories through natural selection quality quantitative descriptions of local climate at successive time (Enright. 1976; Woodward, 1987, 1992; Woodward and Williams, intervals (Hartmann et al., 2002; Miles et al., 2006). Vascular plant 1987). In effect, the genomes of vascular plant species contain indicators may help meet this need, but they may also provide information about past and present climates. If properly calibrated, information regarding the biological impact of changing climate. an assemblage of vascular plants should provide a quantitative Outside of palaeoecology, few researchers have used vascular signal of site-specific climatic conditions. The use of vascular plant plants as indicators for obtaining quantitative estimates of local climate variables: Karlsen and Elvebakk (2003) mapped local temperature variations in East Greenland using an "Index of Therrnophily'', a combination of a plant species' summer temper- ature needs (determined by geographical distribution), its local abundance, local habitat diversity, and the local proportion of plots, which represent a systematic subset of the inventory plot unproductive land. More recently, Garbolino et al. (2007) grid that spans the forestlands of the USA. The density of Phase 3 determined the climate affinities of nearly 2000 vascular plant plots is about one per 390 km2. Each plot consists of a cluster of species in France based on correlations between the geographical four 168 m2 non-overlapping subplots, arranged within a 1 ha distributions of reIeves and meteorological stations. For each of six circle. Each subplot contains three 1 m2 quadrats. Field crews climate variables, they calculated an affinity as a function of a recorded the presence and cover of each species with heights of up species' maximum frequency and the dispersion of its frequencies to 1.8 m on each quadrat. They subsequently searched the subplots around the maximum. Using vascular plants as indicators, the for up to 45 min to find additional vascular plant species of any authors produced detailed climate maps of France. height that occur outside the quadrats. Vegetation sampling of Since the 1970s, many palaeoecologists have employed Phase 3 plots occurs during June, July, or August, and, if fully vascular plants as indicators to derive quantitative climatic implemented, remeasurement would take place on the standard information about local or regional late-Quaternary environments inventory cycle (10 years in the western USA). Stapanian et al. (recent examples include Finsinger et aI., 2007; Li et al., 2007; (1997) and Schulz et al. (2009) provide detailed information about Neumann et al., 2007; Wilmshurst et al., 2007; St.jacques et al., the design and sampling of FIA Phase 3 plots. 2008). These researchers have sought to develop transfer func- Cover values reflect the vagaries of phenology, weather tions, multivariate equations that permit the estimation of ancient conditions, field crew biases, and actual abundance (Gray and climate parameters from data on fossils (Birks, 1995, and Azuma, 2005; Smith et al., 1986; Schulz et al., 2009). As an references therein). Calibration sets, which include spatially alternative, we computed two frequency measures for each species correlated present-day climatic conditions and pollen grain/ present on a given plot. For species i, Fqi is quadrat frequency and pteridophyte spore/phytolith counts, specify the transfer func- FSi is subplot frequency: tions. Formally, C (a matrix of n standardized observations, one at each of n locations on q climate variables), X (the corresponding matrix of standardized observations on p biological variables at each ofthe same n locations), B (a matrix of transfer functions, one for each of the q climate variables, with each comprised of p elements), and 0 (a matrix of random noise, with n parts for each of the q climate variables) are related through a linear model: Both Fqi and FSi vary from 0 to 1. Quadrat frequency and subplot frequency provide rough yardsticks of abundance, not just distribution, of a species within an FIAPhase 3 plot. Adult survival, B acts as a set of transfer functions, because observations on the p fecundity, within-plot dispersal, and the survival of recruits dictate biological variables render estimates of the q climate variables. the frequency of a vascular plant species. While they are not The use of vascular plants to monitor climate carries the synonymous with population density, Fqi and FSi convey informa- assumption that their populations are always in dynamic equilibri- tion about the proportion of a plot occupied by a species. A plant um with climate (Davis and Botkin, 1985; Webb, 1986; Prentice species with a large population, as measured by cover value, et al., 1993): the list of species in a community and their properties should occupy a larger proportion of a plot than a species (e.g., abundances) match those allowable under the prevailing represented by a small population (Andrewartha and Birch, 1954; climatic regime. If the biological response lags significantly behind Krebs, 1985). the pace of climate change, then this assumption is unjustified. Davis Like all frequency measures, Fqi and FSi depend on plot size. and Botkin (1985) showed, through simulation studies, that short- Thus, we excluded from consideration any plot comprised of less term (i.e., decadal) climate changes may have little immediate effect than 100% accessible forest, as it would have sampled fewer than on the adult survival of long-lived tree species. Even annuals and 12 quadrats and fewer than four subplots. short-lived perennials may be little affected by climate change as For construction of the calibration set, we obtained data from long as the buffering capacity of the canopy persists (Smith, 1965; 107 fully forested FIA Phase 3 plots sampled in 2001, 2004, and Davis and Botkin, 1985). Regular disturbance may be necessary to 2005 distributed across Oregon and Washington (Fig. 1). We guarantee that short-term climate change will bring about a calculated quadrat frequencies for 538, and subplot frequencies for measurable species response. 698, vascular plant species on these plots. In view of recent Palaeoecologists use the past remains of vascular plants as findings (Lischke et al., 2002; Miller et al., 2008; Payette, 2007; indicators